At present, the maximum transmission bandwidth of Long-Term Evolution (LTE) systems is 20 MHz. It cannot meet the requirement of high data rate. Currently, in order to increase user's transmission rate, LTE-Advanced is proposed based on LTE. In LTE-A systems, multiple Component Carriers (CCs) are aggregated to obtain a wider bandwidth and form uplink and downlink of the communication system, so as to support higher transmission rate. This technique is called Carrier Aggregation (CA). For example, in order to support 100 MHz bandwidth, five 20 MHz CCs may be aggregated. Herein, each CC is referred to as a cell.
Among multiple downlink CCs configured by a base station, one is a primary cell (Pcell) and others are secondary cells (Scell). The base station configures a UE to receive downlink data of multiple cells through higher layer signaling. The number of cells scheduled in one subframe may be smaller than or equal to the number of cells configured by higher layers. For example, in FIG. 1, four cells are configured, respectively are cell 1 to cell 4. The base station schedules only one cell, i.e., cell 1. For another example, in FIG. 2, four cells are configured, respectively are cell 1 to cell 4. The base station schedules three cells, i.e., cell 1, cell 2 and cell 3. Herein, data transmission of one downlink cell may be scheduled by Physical Downlink Control Channel (PDCCH) transmitted in other cells. This method is referred to as cross-carrier scheduling. Or, the data transmission of one downlink cell may be scheduled by the PDCCH of this cell. This method is referred to as non cross-carrier scheduling.
Based on the CA technique, the base station transmits downlink data to the same UE on multiple cells. Accordingly, the UE needs to feedback HARQ-ACK information of the downlink data transmitted on multiple cells. According to a discuss result of LTE-A, the HARQ-ACK feedback information of the data transmission on the cells is transmitted on one uplink cell (i.e., uplink Pcell). In order to support the transmission of multiple HARQ-ACK feedback bits, a method based on channel selection may be adopted in LTE-A to support at most 4 HARQ-ACK feedback bits. This method has been used in LTE TDD systems. In the case of channel selection of non-transmit-diversity, the number of HARQ-ACK resources to be allocated is equal to that of HARQ-ACK feedback bits.
According to the discuss result of LTE-A, in LTE-A FDD systems, the channel selection method supports at most 2 cells in fact, and each cell may feed back one or two HARQ-ACK feedback bits. Herein, in the case that Spatial Orthogonal Resource Transmit Diversity (SORTD) is not adopted to support transmit diversity, the method for allocating the HARQ-ACK channel resources is as follows.
For a downlink Pcell, the HARQ-ACK channel used by the HARQ-ACK feedback information of the Pcell is determined according to a Control Channel Element (CCE) index of the PDCCH via an implicit method.
For a downlink Scell, if cross-carrier scheduling is not adopted, the HARQ-ACK channel used by the HARQ-ACK feedback information of the Scell is determined according to a HARQ-ACK Resource Indicator (ARI) in the PDCCH scheduling the Scell; if the Scell is cross-carrier scheduled from the PDCCH of the Pcell, the HARQ-ACK channel used by the HARQ-ACK information of the Scell is determined via a implicit method according to the CCE index of the PDCCH.
If the Cell is configured with a Single Input Multiple Output (SIMO) transmission mode, since it is only required to feedback one HARQ-ACK with respect to one Transmission Block (TB) of the Cell, one HARQ-ACK channel needs to be allocated. Accordingly, if the Cell is configured with a Multiple Input Multiple Output (MIMO) transmission mode, two HARQ-ACK need to be feedbacked with respect to two TBs of the Cell. Therefore, two HARQ-ACK channels need to be allocated.